In copending application Ser. No. 845,738, filed Oct. 26, 1977 by Elmer G. Fridrich, titled "Miniature High Pressure Discharge Lamps", similarly assigned, useful and efficient high pressure discharge lamps are disclosed having much smaller sizes than have been considered practical heretofore, namely discharge volumes of one cubic centimeter or less. In preferred form achieving maximum efficacy, these high intensity lamps utilize generally spheroidal thin-walled arc chambers together with vapor pressures above 5 atmospheres and reaching progressively higher levels as the size is reduced. The convective arc instability usually associated with the high pressures utilized is avoided and there is no appreciable hazard from possibility of explosion. Practical designs provide wettage ratings or lamp sizes starting at about 100 watts and going down to less than 10 watts, the lamps having characteristics including color rendition, efficacy, maintenance and life duration making them suitable for general lighting purposes.
High pressure metal vapor lamps have certain inherent shortcomings which persist even in miniature sizes. One of these is the delay in achieving full brilliance after ignition, caused by the need to heat up the envelope and vaporize the metallic fill. This delay may be termed the cold start delay. Another is the even longer delay, termed the hot restart delay, which occurs should there be momentary interruption of power to the lamp. The lamp then becomes extinguished and relighting will not occur immediately upon restoration of power. It is necessary firstly for the lamp to cool down and the metal vapor pressure to diminish before the ballast can restrike the arc, and then more time is required for the lamp to heat up to full brilliance.
It is known to use a separate standby incandescent lamp in combination with a discharge lamp and a control circuit to supplement the light from the discharge lamp during its off or low illumination periods and thereby achieve instant light. Such a system is disclosed in Swiss patent No. 377,937 (Leuenberger, 1964) in which the standby lamp is energized by a relay whose winding receives two oppositely directed voltages derived from the circuit of a mercury vapor lamp. During the cold start interval and also during the hot restart interval, the vector difference of the two voltages is large enough to energize the relay and switch on the standby lamp. During normal operation, the vector difference is too small to energize the relay so that the standby lamp is switched off. Another example is described in Swiss patent No. 444,305 (Vogeli, 1967) wherein the relay is replaced by a silicon controlled rectifier connected in series with the standby lamp across a power supply. Yet other examples are disclosed in U.S. Pat. No. 3,517,254 (McNamara Jr., 1970) which uses a voltage breakdown device such as a diac connected in series with the standby lamp to control the current flow through it, the diac and the standby lamp being connected in parallel with the discharge lamp; and in U.S. Pat. No. 3,737,720 (Willis, 1973) which uses a pair of relays to assure that the standby incandescent lamp is automatically turned on at cold start or at hot restart.
A characteristic of the miniature high pressure metal vapor lamps with which the invention is particularly concerned is the very rapid deionization to which they are subject. In operation on 60 Hz alternating current, deionization is almost complete between half cycles so that a very high restriking voltage is required to be provided by the ballast. Particularly in metal halide lamps during the lamp warm-up interval, the reignition voltage reaches extremely high levels in the first few seconds after arc ignition. Due to these deionization limitations associated with low frequency operation of miniature metal vapor lamps, recourse is being had to high frequency ballasts operating in resonance-free regions in the range of 20 to 50 kHz. In these regions the miniature lamps are not subject to destructive acoustic resonances and stable operation is possible as taught in copending application Ser. No. 864,578 filed Dec. 27, 1977, by John M. Davenport titled "High Frequency Operation of Miniature Metal Vapor Discharge Lamps", assigned to the same assignee as the present invention. The type of circuit favored for such high frequency operation, frequently termed an inverter, in general comprises a power oscillator with current-limiting means coupled to the miniature lamp. The control circuits known to the art for assuring instant light with a discharge lamp by means of an associated auxiliary incandescent lamp or filament are not well suited to the high frequency ballasting circuits favored for miniature high pressure metal vapor lamps.
One object of our invention is to provide an instant light lamp combining a miniature arc tube with a standby filament for providing light immediately when the lamp is switched on. Another object is to provide a ballast and control circuit particularly suitable for high frequency operation of an arc tube together with electronic switching of the filament to achieve instantaneous lighting whether at a cold start or at a hot restart.